176 GEOLOGY OF THE HIGH PLATEAUS. 



ing- the northern summit of the Tushar mass, and occurs in several out- 

 lying knobs and small crests to the east and northeast of Belknap. But the 

 northwestern slope of the range has been mantled by great floods of it, 

 which have poured in massive sheets from summit to base, burying the 

 antecedent topography of the mountain and generating a new one. The 

 individual eruptions making up this rhyolitic mass appear to have been 

 numerous, some very voluminous, others very small. The smaller ones 

 are seen to fill up old ravines and to mold themselves upon uneven pre- 

 existing surfaces, while the grander floods pour over everything and spread 

 out over great expanses of mountain side. Although this lava is, with the 

 exception of a few minor basaltic streams around the western base of the 

 Tushar, the most recent of all the outbreaks, yet absolutely it is of con- 

 siderable antiquity. Since the extinction of the vents from winch it was 

 emitted there has been a long period of erosion. Belknap and Baldy, 

 together with the eastern outliers, are mere remnants of piled-up sheets, 

 which were perhaps once continuous, but are now sepai-ated by profound 

 ravines, which have been excavated by erosion. 



The indications are abundant that the period separating the earliest 

 from the latest eruptions was a very long one. The contact of the earliest 

 liparites with the Jurassic quartzites shows heavy floods of lava pouring 

 over a very uneven surface and piled up in layers by successive eruptions 

 to a thickness of more than 2,000 feet. These, in their turn, show a sub- 

 sequent degradation by erosion not only in the sculpturing- and carving of 

 the beds, producing an unconformity in some of the contacts, but also in 

 the existence of local conglomerates composed of the water-worn fragments 

 of the dilapidated rocks cemented by finer detritus derived from the decom- 

 position of the feldspathic materials. 



These earlier eruptions appear to have been followed by a long period 

 of calm, during which they were attacked by the degrading force and 

 slowly wasted by decay. In many places the beds were cut through down 

 to the quartzite and a fresh topograph}- was carved out by erosion. After- 

 wards the activity was reopened with fresh eruptions of a different charac- 

 ter. These second eruptions were grander than the first, some of the beds 

 being many hundreds of feet in thickness, spreading over great areas, and 



